3 sec vs 1 sec CB Fault Duration

[gianella]

Hi,

Normally MV & HV circuit breakers have 3 and 1 sec fault duration rating. If a CB has a 31.5kA rating for 3sec, is it correct to consider that it can withstand 31.5kA x 1.732 for 1 sec as derived from the formula used for cables (I2t = constant)? Will this formula apply to circuit breakers?

My initial thought is that CBs are type-tested only for the rated fault current at its rated duration. My fear is that a 31.5kA-3s CB may not withstand 54kA fault for 1 sec. Even though normally the CB are designed to trip faster than 1 sec, I don't think it is prudent to use 31.5kA CB in a system with 50kA calculated fault current. Hoping to hear any views on this issue.

 

[ppedUK]

Is this a hypothetical scenario you are presenting?

A similar question was posed in another post with regard to short time withstand rating of switchgear. The withstand rating is NOT the same as Let-Through Energy I2t. You cannot extrapolate the current and time rating for higher fault levels. The fault rating of 31.5KA is the 3phase symmetrical rating. You most definitely cannot multiply that figure by 1.732.

If you really do have a fault level of 50kA, you are at the limit for MV switchgear that will withstand and interupt that fault level, particularly if you want a 3 second rating (it is difficult to find). Are you sure about the 50KA, or is that hypothetical? I have been involved in some very high fault level MV systems with very large capacity transformers, generators etc. interconnected, but that was an unusually large MV system and it posed a great headache for sourcing switchgear.

 

[gianella]

Thanks for your reply ppedUK. The system is actually HV. A large CCGT plant will connect to an HV substation that will increase the fault level to around 50kA. Two connection options are available at nearby 132kV SS and 230kV SS. Both were found to exceed kA rating of existing CBs if the new plant connects to either of them. The initial study showed that existing CBs are almost in the threshold of their kA ratings. Initial recommendation is to replace the existing 132kV CBs & auxiliaries, more than 20 sets!

 

[rbulsara]

I would agree with ppedUK that you need to select the "published" fault current and withstand ratings (duties) and compare to your calculated duties.

I am not sure there is way to calculate the way you describe.

Rafiq Bulsara

http://www.srengineersct.com

 

[gianella]

Hi Rafiq,

This is the formula used for cables: I^2 x t = constant

I2 = I1 x sqrt(t1/t2)

if I1=31.5kA, t1=3s, t2=1s, then I2=54kA

I am more inclined to agree with ppedUK that this cannot be applied to CBs. I just wish to get 2nd opinions on this issue. If you have technical references on this, I would appreciate much.

 

[rbulsara]

I did not say anything different than what you are saying.

Perhaps you can ask some manufacturers and see what they say and post here the answers for everyone's benefit.



Rafiq Bulsara

http://www.srengineersct.com